freelance work visas in Berlin

Protected content am an American, newly living in Berlin. I am looking to get a freelance work visa, to work in the field of real estate. I understand that for a normal work visa I would need to be sponsored by my employer. Does anyone know, for certain, how it works for a freelance visa? How do I go about it? Where do I go? What documents would I need? Any strategies? Any experiences anyone can relay to me? Please let me know...